Re: napi_busy_poll

[Olivier Langlois <olivier@trillion01.com>]

On Tue, 2022-02-15 at 03:37 -0500, Olivier Langlois wrote:
> 
> That being said, I have not been able to make it work yet. For some
> unknown reasons, no valid napi_id is extracted from the sockets added
> to the context so the net_busy_poll function is never called.
> 
> I find that very strange since prior to use io_uring, my code was
> using
> epoll and the busy polling was working fine with my application
> sockets. Something is escaping my comprehension. I must tired and
> this
> will become obvious...
> 
The napi_id values associated with my sockets appear to be in the range
0 < napi_id < MIN_NAPI_ID

from busy_loop.h:
/*		0 - Reserved to indicate value not set
 *     1..NR_CPUS - Reserved for sender_cpu
 *  NR_CPUS+1..~0 - Region available for NAPI IDs
 */
#define MIN_NAPI_ID ((unsigned int)(NR_CPUS + 1))

I have found this:
https://lwn.net/Articles/619862/

hinting that busy_poll may be incompatible with RPS
(Documentation/networking/scaling.rst) that I may have discovered
*AFTER* my epoll -> io_uring transition (I don't recall exactly the
sequence of my learning process).

With my current knowledge, it makes little sense why busy polling would
not be possible with RPS. Also, what exactly is a NAPI device is quite
nebulous to me... Looking into the Intel igb driver code, it seems like
1 NAPI device is created for each interrupt vector/Rx buffer of the
device.

Bottomline, it seems like I have fallen into a new rabbit hole. It may
take me a day or 2 to figure it all... you are welcome to enlight me if
you know a thing or 2 about those topics... I am kinda lost right
now...